Here's a couple. Not the best, but hopefully they'll give you some idea.
Boo is 10 1/2lbs, it's obvious which one he is in the pictures
The first pic is him with Manolo, who is 3 1/2lbs, standing in front of him.
The second pic is him with Gracie Ruth, who is around 3 lbs in this pic.
I don't find the size difference to be at all an issue for 2 reasons. One, Boo is very aware of his larger size and will play only as hard as he needs to "defend" himself. Two, these little ones are fierce! And not at all intimidated by anything. In fact I actually have more trouble with Gracie and Yogi who are only a pound different in size. They go full bore at each other.
A smaller companion for Gus shouldn't be a problem at all. Good luck!